Controlling lighting in Edit Mode
You can control the lighting of the mesh via the user preferences. However if some of the faces are dark and others not, it is a telltale sign of a certain flaw in your mesh: normals pointing inwards. You can fix it by selecting all (A), hitting space and looking for Recalculate Normals.

If you want to read up on normals, see Wikipedia: https://en.m.wikipedia.org/wiki/Normal_(geometry)